As the telecommunications industry and society continue to push for mobile communications devices which are smaller, lighter, less expensive, and more energy efficient, the requirements for bandpass filters within these devices become increasingly stringent. Where once transversely coupled surface acoustic wave (SAW) resonator filters were widely used, high-performance transversal SAW filters or longitudinally coupled SAW or LSAW filters have begun to take their place. Transversal SAW filters have the advantages of high flexibility, wide bandwidth, and flat group delay time. However, with newer digital mobile communications protocols requiring smaller size and even less insertion loss, transversal filters simply cannot meet the requirements.
Longitudinally coupled SAW and LSAW resonator filters have become the technology of choice to meet these requirements because of their wide achievable bandwidth and low insertion loss. LSAW modes are typically employed on piezoelectric materials such as lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) primarily for their high propagation velocity and piezoelectric coupling, as compared to conventional SAW (Rayleigh) modes.
Conventional longitudinally coupled LSAW resonator fitters as described in U.S. Pat. No. 5,485,052 typically consist of a plurality of LSAW resonator filter tracks connected in series. Each track consists of a pair of reflective gratings, between which are disposed a plurality of interdigital transducers (IDTs). In each track, one or more non-adjacent IDTs are connected together electrically so as to form a signal input for the track, and the remainder of the IDTs are electrically connected so as to form an output. Adjacent tracks are connected together in series such that the output of the first track is connected to the input of the second, whose output is connected to the input of the third, etc. The input of the first track and the output of the last track comprise the electrical input and output of the bandpass filter. The most common configurations employ only two tracks with two, three, or five IDTs in each track. FIG. 2a shows a schematic representation of a two-track longitudinally coupled LSAW resonator filter of the prior art with three IDTs per track, and FIG. 2b shows one with five IDTs per track.
Good bandpass characteristics can be achieved with longitudinally coupled LSAW resonator filters by introducing resonant cavities between adjacent IDTs and between the gratings and the IDTs adjacent to them. Typically in the art, the resonant cavities are nothing more than spacers inserted between each IDT and its neighboring IDT or grating. The length of these spacers can be either positive (i.e. moving the IDTs/gratings further apart) or negative (i.e. moving the IDTs/gratings closer together). Spacers between adjacent IDTs are typically on the order of ±λ/4, where λ is the acoustic wavelength, and the spacers between the gratings and the adjacent IDTs are usually much smaller (e.g. ±λ/40).
Factors limiting the ultimate performance of longitudinally coupled LSAW resonator filters include, among others, the piezoelectric coupling coefficient of the substrate, the acoustic energy lost due to reflective scattering into the bulk of the substrate, and the power density at all points within the device. The present invention improves upon all three of these factors, thereby allowing the realization of a longitudinally coupled LSAW resonator filter with improved bandwidth, insertion loss, return loss, and power handling capability over the prior art.